Chilling Out with Cold Plasmas

There are two things people think about when they hear the word "plasma." The first is blood plasma, the liquid part of blood that holds blood cells in suspension. The second, if you love physics, is an ionized gas (if you love geology, you'll think of a bright green chalcedony stone), usually at fairly high temperatures. The sun shoots out plasma arcs, for example. You can find them in plasma TV displays, you can use them to create antennae, and fans of science fiction likely fantasize about shooting them firearms as a high-tech weapon. (Lightning is a form of plasma.)

There are also so-called "cold plasmas." I wrote about this topic back in 2007, both in Physics Today and at Cocktail Party Physics, focusing on their potential to kill bacteria, remove dental plaque, loosen the connections between cells that make up biological tissue, help coagulate blood and reduce bleeding following a wound, or during surgery, and perhaps even remove cancerous tumors. And an October paper in the Journal of Physics D: Applied Physics describes a potentially revolutionary new cold plasma device, similar to a blowtorch, for treating blood cancer leukemia.

"We have a really amazing device," lead author Mounir Laroussi (Old Dominion University) told the folks at Physics Buzz. "We can generate a beam of plasma that is around room temperature. It doesn't burn anything; it doesn't destroy or poke holes. You can touch it with your hand." Laroussi's results are pretty startling: after a mere 10 minutes' exposure to the cold plasma, more than 90% of leukemia cells in the study were destroyed.

Cold plasma blowtorch. Credit: Chuck Thomas, Old Dominion University

The term "cold" can be a bit misleading. (Eg, "high-temperature superconductivity" takes place at temperatures common to liquid nitrogen.) Many cold plasmas are "cold" compared to, say, the sun, but still pretty hot: on the order of 70 to 100 degrees Celsius. Apply that to living human tissue, and it's gonna burn. Badly.

Still, they're useful for things like sterilizing drinking water and decontaminating industrial surfaces. That's because they kill ("inactivate") bacteria by destroying the bacterial cell membrane via a lethal combination of charged particles, free radicals and UV radiation. They work fast, too: the Air Force has an active cold plasma research program, using them to break down the chemicals found in toxins like anthrax in mere minutes, compared to several hours for other methods.

Sometime in the late 1990s, researchers figured out how to create truly room-temperature cold plasmas in the laboratory, so for the first time, they could be tested on biological tissue. And that's the focus of Laroussi's research. Per Physics Buzz:

Scientists create cold plasma by sending super-speedy electrons through gasses like helium and air. These electrons hit the atoms and molecules with so much energy that they pull off the outermost electrons of the atoms and molecules in the gas, creating a soupy mixture of free electrons and free ions. The gas remains at around room temperature, Laroussi explained, because the energy required to separate the electrons from their atoms quickly dissipates, leaving the gas ions cool.

There was an intriguing delayed effect with the plasma blowtorch. While the leukemia cells seemed fine immediately after being blasted with the cold-plasma plume for ten minutes, within four to eight hours they started to die. Laroussi hypothesizes that the plasma plume might trigger a biochemical reaction of sorts, inducing cell death in the leukemia cells while leaving normal cells intact.

According to George Washington University's Michael Keidar, among the molecules in a cold plasma is ozone, which is especially reactive -- hence the effectiveness of cold plasmas in treating bacterial infections. Keidar studies plasma treatments for cancer and thinks that because cancerous cells have higher metabolisms than healthy cells, they have more ozone. So the addition of even more ozone molecules via the cold plasma plume puts the cancer cells over the threshold and triggers cell death, whereas healthy cells can withstand the blast just fine.

Previously, Laroussi developed a helium-filled plasma pencil capable of creating a long plasma plume of 2 to 3 inches, which can kill bacteria on the delicate surface of human skin without damaging the surrounding tissue. Laroussi has used it on e coli bacteria. Other groups working with cold plasma "jet guns" have demonstrated the destruction of salmonella and even a few viruses.

Plasma pencil, courtesy M. Laroussi, Old Dominion University.

Those decontamination properties are incredibly useful in helping accelerate wound healing, which has roughly three stages. There's an inflammatory stage, where everything is red and/or swollen and painful, in which it might seem like little healing is actually taking place -- in fact, it's easy to confuse with actual infection.

But there's all kinds of things going on to prompt the body into the second stage: producing collagen to strengthen the wound. This can take several weeks, depending on the severity of the injury, and thick scars can develop.

The final stage is called the remodeling phase, in which the body gets rid of the excess scar tissue. Sometimes a heavy raised (keloid) scar still remains, if the wound was especially deep and nasty. Being able to kill bacteria reduces the chance of infection, and being able to remove dead cells and replace them with healthy ones can significantly speed up this weeks-long process.

Back in 2010, researchers at the Gamaleya Institute of Epidemiology and Microbiology in Moscow used a cold plasma torch on two common bacteria, Pseudomonas aeruginosa and Staphylococcus aureus, both antibiotic-resistant strains (thanks to a biofilm) that are common in wound infections. Per Discovery News: "After five minutes, the plasma torch killed 99 percent of bacteria grown in a Petri dish, and after ten minutes, it killed 90 percent of bacteria present in the wounds of a rats. And because the torch can be directed at a specific, small area of infection, surrounding tissue is left unharmed."

Eva Stoffels-Adamowicz of Eindhoven University of Technology in the Netherlands developed a handy little device called a plasma needle -- basically a thin tungsten wire about 50 millimeters long, inside a gas-filled quartz tube -- that enables her to precisely remove or manipulate biological cells. She calls it "surgery without cutting." Just drive a voltage through the needle and voila! A small plasma spark is generated at the tip.

Neither the plasma needle nor the plasma pencil are using a cold plasma to do actual cutting. But a company called Peak Surgical has a prototype device called the Plasma Blade that actually uses cold plasmas to cut biological tissue. Surgical scalpels have served us well for a very long time, but while they cut very precisely, they can't control bleeding. There are alternative electrosurgical devices that can do both, but there's usually some accompanying thermal damage to surrounding tissue.

Comments

Welcome to the Scientific American Blog Network, a forum for a diverse and independent set of voices to share news and opinions and discuss issues related to science. For more information see our About page and Guidelines....more